1. Field of the Invention
This invention relates to a developing cylinder which is used in an electrophotographic developing machine assembled in a copying machine, facsimile equipment, laser printer, etc.
It is an object of the present invention to provide a developing cylinder whose magnetic poles effectively separate a developer from the surface of a sleeve in a separation zone in which the developer is collected into a toner box or in a development zone in which the developer is delivered to a photoconductor.
It is another object of the present invention to provide a developing cylinder whose developer separation efficiency in a separation zone is enhanced so that the mixing ratio of carrier and toner and the quantity of electrification of a developer can be maintained constant.
It is still another object of the present invention to provide a developing cylinder whose developing magnetic pole (for delivering a developer from the surface of a sleeve onto the surface of a photoconductor) is improved in magnetic property so that scavenging can be prevented from appearing on a developed image and the quality/density of the image can be enhanced.
2. Description of the Prior Art
An electrophotographic developing machine is assembled in a copying machine, facsimile equipment, laser printer, etc. As shown in FIG. 14, the developing machine is composed fundamentally of a developing cylinder 100, a doctor blade 3, photoconductor 200 and a developer box 4.
The developing cylinder 100 is shown in FIG. 15 in greater detail. A magnet roll 1 is made by passing a metallic shaft through a hollow portion of a cylindrical magnet made of a resin-bonded magnet or sintered magnet or by attaching short rods to both ends of a pillar magnet, this magnet roll 1 being supported inside a cylindrical sleeve 2 in noncontact condition. The doctor blade 3 is disposed in confronting relation to the sleeve 2 of the developing cylinder 100 as to cover the whole width of the sleeve 2 such that a doctor-sleeve gap is left between them.
Since the developing cylinder 100 is put into operation by rotating the sleeve 2 relative to the magnet roll 1, when the sleeve 2 is rotated about the magnet roll 1, the developer held in the box 4 is picked up onto the surface of the sleeve 2 by magnetic force. The developer put on the surface of the sleeve 2 passes through the gap between the sleeve 2 and the doctor blade 3, by which the quantity of developer attached is controlled, and is then transferred onto the photoconductor with an electro static image imprinted thereon.
The developer used in the electrophotographic developing machine is divided into two types: the one-component developer made of synthetic resin particles scatteringly combined with magnetic powder, and the two-component developer made by bonding toner particles made of resin and adhesive to carrier particles (larger in size than the former) made of iron powder.
Where the two-component developer (made by mixing the carrier and the toner) is used, only the electrified toner transfers onto the surface of the photoconductor 200, with the result that the developer remaining on the surface of the sleeve 2 after toner transfer includes the carrier in a larger mixing ratio; therefore, the developer having such a deviated mixing ratio must be collected into the developer box 4 or removed from the surface of the sleeve 2. For this purpose, a magnetic pole for developer separation is provided in the developing cylinder 100.
FIG. 16 shows magnetic poles on the surface of a conventional magnet roll and a distribution of magnetic flux density. Magnetic poles N1 and N2 are of the N-pole type, and S1, S2 and S3 are of the S-pole type. Specifically, a plurality of magnetic poles are arranged in the circumferential direction of the magnet roll 1, each extending in the axial direction. These magnetic poles function to pick up the developer, transfer, deliver to the photoconductor, and collect into the developer box. A separation zone designated by A in FIG. 16 is provided for the purpose of releasing or separating the developer before collecting it into the developer box.
In view of the purpose of developer separation, the magnetic flux density in the separation zone A should be made as weak as possible. Although this requirement is fulfilled by providing a zone of no magnetic field, it is practically impossible to provide such a zone of no magnetic field. Therefore, in the prior art, the separation zone is defined by providing a magnetic pole NO of weak magnetic flux density which is opposite in polarity to the adjacent magnetic poles S2 and S3.
Although having a weak magnetic flux density, the conventional separation zone sometimes attains insufficient separation. The present inventor studied the causes of such defective separation and found that the concept of "distribution of magnetic flux density" traditionally used is not fit to the designing of the magnet roll. Specifically, distribution of magnetic flux density shows only the magnetic flux density in the radial direction on the surface of the magnet roll, and the direction of magnetization (inside the magnet roll) of each magnetic pole is not always in accord with the radial direction (normal direction) of the magnet roll as shown in FIG. 17 in vector notation. Thus, the conclusion is that the distribution of magnetic flux density showing the radial magnetic flux density on the surface of the magnet roll can not be considered equivalent to an actual magnetic working force on the surface of the magnet roll.
That is, the conventional procedure of designing the magnet roll is wrong which is based on the idea of distribution of magnetic flux density or achieved by evaluating the magnetic working force of each magnetic pole of the magnet roll using only the radial magnetic flux density on the surface of the magnet roll.
Although the conclusion that the conventional designing procedure is wrong is deduced from the designing of the separation zone as described above, it can also be applied to the designing of other magnetic poles for separating the developer from the surface of the sleeve and to the designing of a development zone in which the developer is delivered to the photoconductor.